One of the most challenging issues in transporting hydrocarbon fluids over long distances under water is the formation and crystallization of solids inside the production tubing used to transport the hydrocarbons. In addition to oil or gas, the hydrocarbon fluids produced from a well typically contain water, gas, and dissolved solids. The dissolved solids may include waxes, organic salts, and inorganic salts.
For various reasons, a layer of solids may buildup in the production tubing transporting the hydrocarbon fluids. The buildup of solids within the production tubing may lead to a loss or reduction in the flow of oil or gas through the production tubing. The solids may be formed from several different substances. For example, the solids may be hydrates formed from a mixture of gas and water, wax, asphaltenes, or organic and inorganic salts. These solids are dissolved in the production fluid at production temperature and then precipitate from the production fluid at temperatures below the production temperatures or pressure. For example, the dissolved solids may precipitate from the hydrocarbon fluid as a result of a reduction in the temperature of the fluid, such as ice forming in water cooled below the freezing point of water. In addition, a change in pressure of the hydrocarbon fluid can cause the dissolved solids to precipitate from the hydrocarbon fluid. Chemical changes in the hydrocarbon fluid can also cause the dissolved solids to precipitate out of the hydrocarbon fluid.
A reduction or loss of flow caused by the buildup of solids within a flow line may require extensive remediation efforts. For example, a flow line with a buildup of solids may need to be jetted, drilled, or treated with heat or chemicals to dissolve the solids and return the flow line to its original condition. Such remediation efforts are time-consuming and expensive.
Several approaches have been attempted to overcome this problem. For example, several technologies have been developed that either heat the flow line for production fluid or insulate the flow lines in an attempt to keep the temperature and pressure of the production fluid out of the region where the formation of solids may occur.
An alternative approach that has been attempted is to accept that there will be heat and pressure losses associated with flowing production fluid through production tubing extending along the seafloor and to try to control the process of the formation of solids within the flow line. This solution generally is referred to as “cold flow” or “sub-cooled flow” technology. In cold flow systems, the production fluid is cooled at an upstream location to a temperature where solids will precipitate from the production fluid. From there, the solids are transported with the production fluid as slurry. A slurry is a suspension of solids in a liquid. The solids in the slurry are less likely to buildup in the flow line than are solids that precipitate out of the fluid at various points along the flow line.
There are several drawbacks and problems associated with existing cold-flow systems. Primarily, these systems are complex, unstable, and provide unsatisfactory results. In particular, controlling the formation of solids so that the solids do not buildup in the system or flow line has proven to be difficult.
Therefore, a more effective technique is desired for providing a cold-flow of hydrocarbon products. In particular, a cold-flow technique is desired that would enable a hydrocarbon slurry to be formed in a subsea flow line and transported to a desired destination without adhesion to the walls of the flow line.